1. Field of the Invention
This invention relates to an image forming apparatus for use in an electrophotographic apparatus, an electrostatic recording apparatus or the like.
2. Related Background Art
In an image forming apparatus utilizing the electrophotographic process, an electrostatic latent image is formed on an image bearing member such as a photosensitive drum, and this electrostatic latent image is developed and visualized by the use of a developer, and the thus obtained visible image (toner image) is transferred to a transfer material to thereby obtain an image.
As developing methods, there are generally a monocomponent developing method using a monocomponent developer comprising only a toner, and a two-component developing method using a two-component developer comprising a magnetic carrier (magnetic particles) and a toner, but the monocomponent developing method is simpler in the construction of a developing device and easier in maintenance and therefore, many developing devices based on the monocomponent developing method have been proposed.
Japanese Patent Application Laid-Open No. 58-116559 proposes a non-magnetic monocomponent developing method of effecting development by the use of not a magnetic toner but a non-magnetic toner, and according to this developing method, it is possible to obtain color images satisfying the recent requirement for the quality of image and moreover, a low-cost and compact developing device is realized.
FIG. 9 of the accompanying drawings shows a developing device using the non-magnetic monocomponent developing method which is installed in an image forming apparatus. This developing device E has a developing roller 1 rotatively driven in the direction of the arrow as a developer bearing member in the opening portion of a developer container 3 containing a non-magnetic toner (a non-magnetic monocomponent developer) therein, and a toner supplying and collecting roller 19 as a developer supplying and collecting member and a regulating blade 20 as a developer regulating member abut against the developing roller 1.
The toner supplying and collecting roller 19 comprises a mandrel 19a of SUS or like material and an elastic member 19b of urethane foam or like material covering the outer peripheral surface of the mandrel 19. The roller 19 elastically abuts against the surface of the developing roller 1 by the elastic member 19b and rotates to thereby supply a non-magnetic toner contained in the developer container 3 to the surface of the developing roller 1, and also scrapes any unused toner returned to the developer container 3 with the rotation of the developing roller 1 without being used in development from the surface of the developing roller 1.
The regulating blade 20 comprises a support member 20a formed of phosphor bronze or the like and an elastic member 20b of urethane rubber or like material adhesively secured to the support member 20a. The blade 20 acts to elastically abut against the surface of the developing roller 1 by the elastic member 20b and regulates the toner borne on the developing roller 1 to thereby form a thin layer of toner and impart charges to the toner.
An image forming apparatus using the above-described developing device E will now be described with reference to FIG. 10 of the accompanying drawings. As shown in FIG. 10, the image forming apparatus has a photosensitive drum 11 as an image bearing member rotatively driven at a predetermined peripheral speed in the direction of the arrow, and a charging device 16, an exposing device 15, the developing device E and a cleaning device 18 are disposed around the photosensitive drum 11.
To form an image, the photosensitive drum 11 is first rotated and in the rotation process thereof, the surface of the photosensitive drum 11 is uniformly charged to a desired polarity and potential by the charging device 16, and then image exposure conforming to desired image data is effected by the exposing device 15 to thereby form an electrostatic latent image on the surface of the photosensitive drum 11. Next, in a developing portion wherein the photosensitive drum 11 and the developing roller 1 of the developing device E are opposed to each other, the electrostatic latent image on the photosensitive drum 11 is developed by the non-magnetic toner on the developing roller 1 to thereby form on the photosensitive drum 11 a toner image conforming to the electrostatic latent image.
A transfer bias is then applied to a transfer roller 13 to thereby transfer this toner image to a transfer material P conveyed to a transfer portion in which the photosensitive drum 11 and the transfer roller 13 are opposed to each other. Thereafter, the transfer material P is introduced into a fixing device 14, where the toner image is heated and fixed on the transfer material P, whereby an image is obtained on the transfer material P, thus completing a series of image forming steps.
After the transfer, the photosensitive drum 11 has any untransferred toner residual on its surface without being transferred removed and collected by the cleaning device 18, and thereafter is used again for the image forming steps of charging, exposing, developing, etc. similar to those described above.
In recent years, the further downsizing and lower cost of the above-described image forming apparatus using the non-magnetic monocomponent developing method have been desired and the shortening of the image forming steps (including the omission of some of the steps) has been variously studied.
There has been proposed a cleanerless image forming apparatus in which of the above-described image forming steps, the cleaning step is omitted, but if the cleaning device 18 is simply omitted, there have been the following problems.
If the cleaning means for cleaning the surface of the photosensitive drum 1 is absent, untransferred toner, paper powder, etc. accumulate on the photosensitive drum and prevents uniform charging and image exposure for forming the electrostatic latent image and therefore, the unevenness of potential occurs to the electrostatic latent image and the unevenness of the density of an output image occurs.
Also, the developing device E effects the supply and collection of the toner to the developing roller 1 by the toner supplying and collecting roller 19 abutting against and frictionally rotating with the developing roller 1, and effects the imparting of charges to the toner by the contact friction when the toner passes the regulating blade 13 and therefore, toner in the developer container 3 is very great in a mechanical load applied thereto until it is used for the development on the photosensitive drum 11, and the damage to the toner is very great as compared with the other developing methods.
Further, depending on the disposed position and the direction of rotation of the toner supplying and collecting roller 19, the toner which did not contribute to the development cannot be completely collected and in some cases, the toner has been remaining on the developing roller 1. The toner residual on the developing roller 1 again passes the regulating blade 20 and is carried to a developing area, and if such a step is continuously repeated, an extraneous additive or the like controlling the charging amount and fluidity of the toner is buried into the interior of the toner by mechanical friction and the accumulation of heat, and it happens that the toner becomes a deteriorated toner of which the desired charging characteristic and fluidity cannot be obtained.
This deteriorated toner causes many problems during the image forming process. For example, when the deteriorated toner contributes to development, a proper developing characteristic is not obtained and therefore, bad development occurs or bad transfer such as a hollow image is sometimes caused. Further, the supply of fresh toner onto the developing roller 1 is hindered and the amount of toner coat (the applied amount) may be reduced to thereby cause bad density or the like. Also, when the deteriorated toner is fused on the nip portion of the regulating blade 20 and the surface of the developing roller 1, bad coat such as streaks occurs and also, the imparting of charges to the toner newly supplied onto the developing roller is hindered. Thus, uncharged toner is carried to the developing portion and a bad image such as fog or unevenness may be caused.
The above-described non-magnetic monocomponent developing method is not only great in the load applied to the toner, but also is great in the load applied to the developing device E itself, and when the sponge roller as described above is used as the toner supplying and collecting roller 19, if the frictional contact thereof with the developing roller 1 continues for a long time, the role thereof as the toner supplying and collecting roller will become insufficient due to the friction and damage of the developing roller itself and the clogging or the like of the toner, and good toner supply and collection may become impossible.
As described above, the monocomponent developing method using the non-magnetic toner is simple in the construction of the developing device and can accomplish good development, but is great in the load applied to the toner and the developing device, and is remarkably deficient in long-term stability and durability as compared with the magnetic monocomponent developing method and the two-component developing method. Accordingly, the non-magnetic monocomponent developing method is utilized chiefly in developing devices of the cartridge type in which the developing device is bodily interchanged during the replenishment of the toner supply, and is not so much adopted in developing devices of the type in which the toner supply is replenished as in a copier.
Further, in recent years, with a view to reduce power consumption, the development of a toner which can be fixed at a lower temperature has been progressed, and along therewith, a developing process of low stress corresponding to the toner for low temperature fixation is expected, but there is not yet available a non-magnetic monocomponent developing method provided with such a characteristic.